The production of spherical, solid microspheres of carbamide-aldehyde condensation products having diameters within the range of from about 0.5 to 5 microns for use as opacifyingagents is known in the art. Suitable methods of production are, for example, disclosed in U.S. Pat. No. 3,928,272 of Brancato et al and in U.S. Application Ser. No. 508,322 of David N. Vincent et al.
These microspheres may be produced from an aqueous, solution of partially condensed aldehye condensation product, such as urea-formaldehyde, oily non-solvent for the condensation product and surface-active emulsifying agent. After their high-shear admixture to produce a water-in-oil emulsion, an acid catalyst is added to polymerize the condensation product into substantially spherical, opaque and solid microparticles.
As is also disclosed in the aforementioned application of Vincent et al, it has been discovered that these microparticles may be agglomerated to yield agents of improved opacifying power. This may be accomplished by converting the microparticle medium from a water-in-oil to an oil-in-water emulsion and then providing conditions of high-shear agitation and heating effective to remove the residual oily material and bond the microspheres into spherical agglomerates. These agglomerates are believed to be produced by post-curing and fusion of the initial, polymerized particles after their inversion in form from water-in-oil to oil-in-water emulsion droplets.
These secondary or agglomerate particles are irregular and bumpy, being a spherical shell of the initial or primary polymer microparticles. In contrast to the smooth microparticle precursors having average diameters of between about 0.5 to 5 microns, these agglomerates possess average particle diameters between about 1 to 20 microns, preferably between about 2 to 7 microns and exhibit, by virtue of their pebbled surface, a raspberry-like appearance. These agglomerates are hollow instead of solid.
The most unique characteristic of these agglomerates is that they exhibit an unexpected two to three fold increase in their light scattering abilities as compared to the individual, initial microparticles of which they are composed. This increase in light scattering ability constitutes a substantial advantage in their use as opacifying agents for such diverse environments as paints, paper, plastic, fabric, textile webs and the like to which they may be added in conventional manner.